The Plastid Lipase PLIP1 Is Critical for Seed Viability in diacylglycerol acyltransferase1 Mutant Seed

In developing Arabidopsis (Arabidopsis thaliana) seeds, the synthesis of triacylglycerol (TAG) is mediated primarily by the acyl-CoA-dependent enzyme diacylglycerol acyltransferasel (DGAT1). In the absence of DGAT1 activity, phospholipid:diacylglycerol acyltransferase (PDAT1) plays an important role in TAG synthesis, consistent with the higherthan-expected oil content and altered fatty acid composition of dgatl seed. Transcript profiling of developing wild type (Columbia-0) and dgat1-1 mutant seed identified 602 differentially expressed genes. Expression of genes important for the formation of phosphatidylcholine, including LYSOPHOSPHATIDYLCHOLINE ACYLTRANSFERASE2, and REDUCED OLEATE DESATURATION1 were strongly upregulated, consistent with increased substrate supply for PDAT1. In addition, several genes lacking a defined role in TAG biosynthesis were also upregulated, including the alpha/beta-hydrolase family gene PLIP1, which encodes a plastid-localized lipase. In most tissues, PLIP1 was expressed at equivalent levels in wild-type and dgatl plants, except for developing seed, where transcript levels were higher in the dgatl mutant. Seeds from plipl mutant plants possessed a 20% reduction in oil content and were smaller than seed from wild-type plants. Crosses between dgatl and plipl failed to generate double-homozygous mutant plants. Reciprocal crossing with wild-type plants demonstrated that both male and female gametophytes could transmit the dgatl plipl double-mutant genotype. Double-homozygous dgatl plipl seed formed but was green and failed to germinate. The synthetic lethal phenotype of dgatl with plipl indicates an important role for PLIP1 in the absence of DGAT1 activity, likely by supplying polyunsaturated fatty acid substrates for PDAT1.